Fuel efficiency of many types of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermodynamic efficiency through the use of a smaller displacement when full torque is not required. The most common method of varying the displacement of an engine involves deactivating a group of cylinders substantially simultaneously. In this approach, no fuel is delivered to the deactivated cylinders and their associated intake and exhaust valves are kept closed as long as the cylinders remain deactivated.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously and remain deactivated as long as the engine remains in the same variable displacement mode. Thus, the sequence of specific cylinders firings will always be exactly the same for each engine cycle during operation in a variable displacement mode (so long as the engine remains in the same displacement mode), whereas that is often not the case during skip fire operation. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it operates using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed mode displacements. In contrast, during skip fire engine operation, the engine is not constrained to fire the same set of cylinders each engine cycle. For example, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns.
Many skip fire controllers are arranged to provide a set of available firing patterns, sequences or firing fractions. In some circumstances the set of available firing patterns or fractions will vary as a function of various operating parameters such as engine load, engine speed and transmission gear. Typically the available firing patterns are selected, in part, based on their NVH characteristics. Transitions between firing fraction levels must be managed to avoid unacceptable NVH during the transition. In particular, changes in the firing fraction must be coordinated with other engine actuators to achieve smooth firing fraction transitions.
The Applicant has developed a technology referred to as dynamic skip fire in which firing decisions are made on a cylinder firing opportunity by cylinder firing opportunity basis. Various aspects of dynamic skip fire are described in a number of patents including U.S. Pat. Nos. 7,954,474, 7,886,715, 7,849,835, 7,577,511, 8,099,224, 8,131,445, 8,131,447, 8,616,181, 8,701,628, 9,086,020 9,328,672, 9,387,849, 9,399,964, 9,512,794, 9,745,905, and others, each of which is incorporated herein by reference.
In some applications referred to as multi-level skip fire, individual working cycles that are fired may be purposely operated at different cylinder outputs levels—that is, using purposefully different air charge and corresponding fueling levels. By way of example, U.S. Pat. No. 9,399,964 (which is incorporated herein by reference) describes some such approaches.
The individual cylinder control concepts used in dynamic skip fire can also be applied to dynamic multi-charge level engine operation in which all cylinders are fired, but individual working cycles are purposely operated at different cylinder output levels. Dynamic skip fire and dynamic multi-charge level engine operation may collectively be considered different types of cylinder output level modulation engine operation in which the output of each working cycle (e.g., skip/fire, high/low, skip/high/low, etc.) is dynamically determined during operation of the engine, typically on an individual cylinder working cycle by working cycle (firing opportunity by firing opportunity) basis.
When the use of multiple non-zero firing levels is contemplated (e.g., during multi-level skip fire or multi-charge level operation of an engine), it is often efficient to consider an effective firing fraction which correlates to the percentage or fraction of the cylinders that would be fired at a high or reference output. For example, if half of the cylinders are fired at a cylinder output level of 70% of a full firing output and the other half are fired at the full firing output level, then the effective firing fraction would be 85%. In another example, if a quarter of the cylinders are fired at a cylinder output level of 70% of a full firing output, another quarter are fired at the full firing output level, and the other half are skipped, then the effective firing fraction would be 42.5%. In yet another example, if traditional skip fire operation is used (i.e., firing a designated percentage of the firing opportunities), then the effective firing fraction may represent the percentage of the cylinders that are actually fired.
We have observed that during skip fire operation of an engine, undesirable NVH can sometimes occur during transmission shifts—particularly when the effective firing fraction changes during the transmission shift. The present application describes a number of control schemes that can help mitigate undesirable NVH associated with firing fraction changes that occur in association with, or temporally in the vicinity of, a gear shift.